Apparatus and methods for minimizing as-deposited stress in tungsten silicide films

ABSTRACT

Processing of substrates in a CVD reactor system wherein tungsten silicide is deposited is accomplished with preflow and postflow of reducing gases before and after deposition steps to ensure that tungsten-rich film is not deposited at the interface of the tungsten silicide film to the substrates or on the tungsten silicide film at the end of deposition processing. For systems having a remote gas injection and flow control system connected by a gas supply manifold to a CVD reactor chamber, an isolation valve is provided in the gas supply manifold, and the valve is held closed during at least a portion of time between deposition sequences.

FIELD OF THE INVENTION

The present invention is in the area of methods and apparatus forprocessing wafers as a step in manufacturing integrated circuits (ICs),and pertains in particular to chemical vapor deposition (CVD) andplasma-enhanced chemical vapor deposition (PECVD) of transitional metalswith silicon.

BACKGROUND OF THE INVENTION

Manufacturing of integrated circuits is historically a procedure offorming thin films and layers of various materials on wafers of basesemiconductor material, and then selectively removing areas of the filmsto provide structures and circuitry. Doped silicon is a typical basewafer material, and in various process schemes, metal layers are formedon the doped silicon or on polysilicon or silicon oxide formed from thebase material.

It is well-known in the art that there are certain properties of thinfilms that are more-or-less universally desirable. For example, it isdesirable that applied films in semiconductor manufacturing, andgenerally in all sorts of film deposition, exhibit good adhesion to thesurfaces they are applied upon. Another generally desirablecharacteristic, related to adhesion, is low as-deposited film stress.Although many films are annealed after deposition, the temperature andtime required for annealing, and even the stress level to which a filmmay be reduced by annealing, may be strongly effected by theas-deposited film stress. Also, highly stressed films may deform andseparate from the underlying layers before annealing can be effectivelyaccomplished.

There are a number of well-developed technologies for deposition ofmaterials in the ultra-thin layers required for IC fabrication schemes.The deposition techniques can be roughly classed as either physicalvapor deposition (PVD) or Chemical Vapor Deposition (CVD) techniques.PVD processes include such processes as evaporation and re-condensation,wherein a material, typically a metal, is heated to a temperature atwhich the metal melts and vaporizes. The metal then condenses onsurfaces generally in line-of-sight of the evaporation, forming a film.

Another PVD process is the well-known sputtering process, wherein aplasma of usually an inert gas is formed near a target material, and thetarget is biased to attract ions from the plasma to bombard the target.Atoms of the target material are dislodged by momentum transfer, andform an atomic flux of particles which coalesce on surrounding surfacesgenerally in line-of-sight of the target surface eroded by thesputtering process.

PVD processes have distinct advantages for some processes, such as highrate of deposition, and relatively simple coating apparatus. There aredrawbacks as well, notably an inherent inability to provide adequatestep coverage. That is, on surfaces having concavities as a result ofprevious coating and etching steps, PVD processes are heir to shadowingeffects resulting in local non-uniformity of coating thickness. Thisproblem has grown in importance as device density has increased anddevice geometry has diminished in size.

CVD processes comprise deposition from gases injected into a processingchamber, typically at very low pressure compared to atmosphericpressure. In these processes, one or more materials that are componentsof one or more gaseous precursors are caused to deposit on a substratethrough chemical decomposition and/or recombination. Energy input byheat, sometimes augmented by plasma power are used to drive the chemicalreactions that result in deposition.

Many materials may be deposited by CVD techniques, but the field islimited to those materials which may be introduced to a chamber aseither a gas or a vapor. For example, a film of metallic tungsten may bedeposited on a heated substrate surface by flowing tungsten hexafluoride(WF₆) to the surface in conjunction with a reducing gas, such ahydrogen. The resulting chemical reaction at a hot substrate surfacereduces the WF₆, leaving a film of tungsten on the substrate andproducing HF gas. Tungsten is used in semiconductor manufacturing as acontact film between transistor gates and interconnect traces.

In other well-known CVD processes silicon is provided as silane (SiH₄),disilane (Si₂ H₆), or as dichlorosilane (SiH₂ Cl₂) along with WF₆ toproduce a film of tungsten silicide (W_(x) Si_(y)), which is a preferredfilm for contacts. The present invention has particular relevance totungsten silicide films.

Low resistivity for electrical contacts at devices in semiconductorcircuitry, and low film stress are both highly desirable characteristicsfor CVD deposited tungsten silicide films. Unfortunately, depositionconditions that promote low resistivity do not necessarily promote lowstress, and vice-versa.

Resistivity and film stress before and after anneal depend on a numberof variables, such as hearth and substrate temperature in a CVD reactor,plasma power (if used), silicon-to-tungsten ratio in the as-depositedfilm, chamber pressure during deposition, flow rates and ratio of gasesduring deposition steps, anneal time and temperature, and much more.These variables and the results of varying them are relatively wellknown in the art after several years of depositing tungsten silicide,and competitive edges in general are typically accomplished by differentmanufacturers of CVD reactors and by manufacturers of integratedcircuits (ICs), which manufacturers use the CVD reactors, by developingmethods and hardware for precisely controlling the variables.

Still, even though the variables are well-known and much experience hasbeen gained by many with skill in the art, and many patents have beenawarded, there are still subtle interdependencies among variables thatremain to be thoroughly understood, and causes and effects that areperhaps not as well understood as previously supposed in the art. Thepresent inventors believe they have discovered more than one suchcircumstance in the deposition of tungsten silicide, and have developedmethods and apparatus to provide more desirable films under morefavorable circumstances than were previously thought to be possible.

The process effects that the present inventors have discovered anddocumented, and the steps taken and the apparatus developed as a result,providing new and better equipment and processes, are detailed in fullbelow, forming the basis of the embodiments of the present invention.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention a method is providedfor depositing a tungsten silicide film on a substrate in a CVD reactorchamber, comprising steps of (a) with the CVD reactor chamber closed andthe substrate in position and heated, establishing a preflow of reducergas to the reactor chamber, thereby preventing preformation oftungsten-rich film on the substrate before planned process steps begin;(b) introducing WF₆ and other process gases into the reactor chamber andaccomplishing deposition of the tungsten silicide film on the substrateby the planned process steps; and (c) ceasing flow of the process gasesto stop the deposition of the tungsten silicide film.

In an alternative embodiment a step for postflowing the reducer gasafter the step for ceasing flow of the process gases to stop thedeposition of the tungsten silicide film prevents post-formation oftungsten-rich film on the tungsten silicide film after the plannedprocess steps are complete. In some embodiments the CVD reactor chamberis connected by one or more manifolds to a gas injection and flowcontrol subsystem, and practice of the invention further comprisesclosing an isolation valve between the CVD reactor chamber and the gasinjection and flow control subsystem for substantially all of the timebetween the end of deposition steps for one wafer and the beginning ofdeposition steps on another wafer. The various embodiments of theinvention have particular application in processes for manufacturingintegrated circuits.

In various embodiments of the methods taught preflow, postflow, andisolation may be used individually or in any combination ascircumstances may determine.

In one aspect of the invention a CVD reactor system is providedcomprising: a CVD reactor chamber having a hearth for heating asubstrate to be coated; a gas injection and flow control systemconnected to the CVD reactor chamber by a gas supply manifold; and anisolation valve positioned to isolate the CVD reactor chamber from thegas injection and flow control system. The CVD depositions are performedon a substrate for a first period of time, wherein deposition issuspended while substrates are transferred to and from the CVD reactorchamber during a second period of time, and the isolation valve is heldclosed, isolating the CVD reactor chamber from the gas injection andflow control system during a portion of the second period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an idealized elevation section view of a single wafer CVDreactor and a gas supply system according to an embodiment of thepresent invention.

FIG. 2 is an idealized cross section of a tungsten silicide film on asubstrate, illustrating a phenomenon discovered by the presentinventors.

FIG. 3 is a process flow diagram depicting steps in practice of anembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is well-known that there are several commonalties in CVD processes.For example, virtually all commercial CVD processes, including those forpreparing films of tungsten silicide, are performed at a total pressurein the processing chamber well below atmospheric pressure. Typically achamber that is adapted to be hermetically sealed is employed, andfitted with apparatus for heating substrates to be coated. Apparatus isprovided as well for flowing process gases into the chamber whilesubstrates are present and heated, so films are deposited.

In some processes energy is added by plasma enhancement as well as byheat, and apparatus for striking and maintaining a plasma is provided,and certain gases may be added to the process mix, such as inert Argon,for example, to enhance the provision of a plasma.

Some CVD reactors are batch reactors, in which several substrates areprocessed at the same time, and some are single-substrate reactors,which are typically used with machines called cluster tools, whereinsubstrates (also termed wafers or platters) are moved sequentiallythrough several single-substrate chambers, which are isolated duringprocess steps.

In either the single substrate chambers or the batch systems, betweenprocess cycles the CVD reactor chamber is pumped to a relatively highvacuum level, that is, to a much lower pressure than during processing.For example, in a typical deposition process the pressure in theprocessing chamber may be at 500 milliTorr, while during pumpout thepressure will be reduced to below 10 milliTorr. The purpose of such apump out is to remove process gases from the chamber before a wafer (orwafers) is removed after processing. This ensures (it has been stronglybelieved) that the deposition process is terminated, and allows residualgases to be pumped away before a new wafer (or wafers) is processed.

A typical process cycle, somewhat simplified but applicable to bothbatch reactors and single-substrate reactors, and assuming a startingpoint just at the completion of a deposition, comprises steps of: (1)discontinue flow of process gases to the reactor (2) pump out chamber tohigh vacuum; (3) open transfer valve to air lock or transfer volumeunder vacuum; (4) remove processed wafer or wafers; (5) insert new,unprocessed wafer or wafers; (6) follow processing steps, comprisingestablishing flow of process gases for predetermined periods of time toaccomplish process. If pumping speed is not variable, pumpout (step 2)occurs simply because gas flow is interrupted in step 1. More often,however, one or more throttle valves in vacuum pumping lines may beopened after gas flow is stopped to decrease the time needed forpumpout.

Typically in this generic process flow, heat to a hearth upon whichwafers to be coated are rested (to heat the wafers), is continued duringthe pumpout phase. This is true for several reasons, among them thatprecise temperature control is considered critical, and tuning power onand off to the hearth would cause hard-to-control temperatureperturbations. Another is that the hearth typically has considerablethermal mass, and temperature can therefore be changed only relativelyslowly.

From this point in the specification descriptions are based on a singlewafer reactor of a type meant to be adapted to a cluster tool machine,even though many embodiments of the invention pertain and apply to CVDsystems of other sorts. It is assumed that the descriptions are alsoapplicable to batch reactors. In cases where this is not so, specificreference is made to the reactor type.

Consequent to the generic process sequence listed above, a wafer forwhich processing is finished by virtue of the deposition steps fortungsten silicide (in this instance) being completed, is kept atprocessing temperature during pumpout, before the valve to the transfervolume is opened. Moreover, a new wafer brought into the reactor fromthe transfer volume and placed on the hearth is heated to processingtemperature before process gases are flowed into the reactor.

The present inventors have discovered that some of the assumptions sofar relied upon by developers and researchers are not entirely correct.Workers in the art have assumed, for example, that deposition isterminated by discontinuing flow of process gases to a CVD reactor andpumping the chamber to a pressure level much lower than the processingpressure. The inventors note, as well, that there is no such thing as aperfect vacuum. Vacuum levels are relative. Moreover, pumping speed isalways finite, so pumpout is done over a time period. Further, a waferfor which process is ostensibly finished is still on the hearth andheated as pumpout is accomplished.

The present inventors are aware also that WF₆ is a relatively wet gas,in the sense that it tends to adhere to the inner surfaces of manifolds,chamber walls, and other surfaces. The present inventors theorizedtherefore, that even if flow of process gases were to be suspended andpumping speed were to be increased to accomplish pumpout, there could bestill sufficient WF₆ available in the process chamber to continue somedeposition of tungsten or very tungsten-rich film on the wafer on theheated hearth. In this case a thin layer of tungsten-rich film could bedeposited on the surface of a just-prepared tungsten silicide film, andif that were true, the thin tungsten-rich film (which might be puretungsten) could measurably effect film characteristics, such asas-deposited resistivity and stress.

The present inventors have tested their theory and found it to be true,and the conventional wisdom to be flawed. Moreover, the same effect hasbeen found at the beginning of a process cycle, after a new wafer isplaced on a hearth in a processing chamber, and before process gases areflowed into the chamber. What actually takes place under certainconditions is that a thin, tungsten-rich film is formed on a waferbefore planned processing commences, and another thin, tungsten-richfilm is formed on the wafer after processing is thought by others to becomplete.

For purposes of illustration of both prior art and the presentinvention, FIG. 1 is an idealized elevation cross section of asingle-wafer CVD reactor according to an embodiment of the presentinvention. In FIG. 1 a single wafer CVD reaction chamber 11 includes aheated hearth 13 upon which a wafer 15 is placed for processing, apumping port 21 through which gases are evacuated (this port in mostinstances may be throttled to control pumping speed), a transfer port 28through which wafers are inserted to and removed from the reactor(usually to an intermediate area held at relatively high vacuum), andboth a showerhead manifold 17 and a ring manifold 19 for injectingprocess gases into the CVD reactor. It will be apparent to those withskill in the art that this is a simplified illustration, and there aremany conventional elements not shown.

CVD reactor 11 is connected by a gas supply manifold 23 to a gasinjection unit 25, which includes gas reservoirs, valves, mass flowcontrollers, and the like for injecting and mixing gases to be suppliedto reactor 11 to accomplish deposition processes. This manifolding isrelatively complicated for a number of reasons, among them that some ofthe gases used are very toxic and must be kept remote from otherequipment, and the fact that some gases can be premixed, and because ofgas phase reactions other gases cannot be premixed.

FIG. 2 is an idealized cross-section of a tungsten silicide film 29 asdeposited in a CVD reactor on a substrate 31. The inventors havediscovered that in many, if not most commercial processes a very thinlayer (33, 35) (typically a few atomic diameters thick, but varying inthickness from case to case) of pure tungsten or tungsten-rich film isfound on both sides of the intentionally deposited tungsten silicidelayer 29 for wafers processed in commercially-available reactors. Theinventors also discovered the tungsten-rich films were in some casesmore prominent than others, and the inventors were successful inisolating the causative factors. Further, the tungsten-rich films havebeen correlated to as-deposited stress in particular. The presence ofthese tungsten-rich films, either on the substrate side or the open sideof the tungsten silicide film, or on both sides, has been demonstratedto measurably increase as-deposited stress.

By diligent experimental investigation the authors have isolated thesources of the tungsten-rich films on both sides of intentionallydeposited films. The facts are these:

1. When a wafer is placed on a heated hearth prior to a process cycle,and the transfer door is closed, the wafer, being of small thermal masscompared to the hearth, heats rapidly to process temperature. Moreover,in reactors used with WF₆, the gas is still present in the supposedlypumped-out chamber at a finite partial pressure. This is because WF₆,like water vapor, clings to manifold and chamber surfaces and is onlyslowly volatilized and pumped away. This is a phenomenon known in theart as outgasing, and is well-known relative to, for example, watervapor. As a result of the presence of WF₆ and adequate wafertemperature, a thin layer of tungsten or very tungsten-rich film isdeposited before planned deposition steps are initiated.

2. At the end of planned deposition steps, when process gases are shutoff and pumpout is accomplished, WF₆ is again present by residual in thereactor, and by outgasing from surfaces in manifolds and the reactorchamber, and another thin tungsten-rich film is deposited on top of theintended tungsten silicide film.

3. If, in initiating the planned processing steps, WF₆ and the reducinggas (for example silane, disilane, dichlorosilane) are turned on at thesame time, there is a finite possibility that a small amount of WF₆ willreach the wafer surface before the reducing gas, and the initialtungsten-rich film will be even more substantial.

4. If, at the end of the planned processing steps, WF₆ and the reducinggas are shut off at the same time, there is a finite possibility thatWF₆ will remain in the chamber longer than the reducing gas, or that theratio of WF₆ to the reducing gas will be much higher for a time thanexpected, and the post-process tungsten-rich film will be even moresubstantial.

The present inventors have, by film analysis techniques and experiment,verified the presence of the pre- and post-process tungsten-rich films,and the effect of these unwanted films on as-deposited filmcharacteristics. Methods and apparatus for avoiding these tungsten-richlayers and thereby enhancing as deposited film characteristics arepresented below.

The system of FIG. 1, described in some respects above, has, in additionto those elements previously discussed, a shut-off valve 27 in the gassupply line after the injection and mass flow equipment. In someembodiments there is also a valved inlet 26 downstream from theisolation valve 27. Because the manifolding within the gas injectionunit 25 is of necessity rather lengthy and complicated, there isconsiderable surface area upon which WF₆ may adsorb. By closing valve 27after process is complete, and keeping it closed during all of the timeof pump out and wafer transfer through port 28, the inventors havediscovered that the pre- and post process tungsten-rich films areminimized. The as-deposited stress level for films deposited underconditions wherein valve 27 is used as described is found to besignificantly less than when the valve is used. For example, in aprocess for depositing a tungsten silicide film using dichlorosilane asthe reducing gas, the as-deposited stress measured in a system wherein ashut-off valve such as valve 27 is not used was measured as >1.4E10dynes/cm², while in a similar system wherein a shut-off valve such asvalve 27 is used, the as-deposited stress was measured as <1E10dynes/cm².

FIG. 3 is a flow diagram illustrating practice of the present inventionin combined embodiments. The steps in a cycle may be described from anyconvenient point through an entire cycle, as the cycle sequence isrepeated. For convenience description of FIG. 3 begins at the point(step 37) where planned process steps are complete. At this point gasflow is suspended, except for reducing gas. Reducing gas flow iscontinued to ensure a high ratio of reducing gas to any residual WF₆ inthe reactor, ensuring that any continued deposition is nottungsten-rich.

In some cases a downstream reducing gas valved inlet (see element 26 ofFIG. 1) is provided, in which case isolation valve 27 is closed andvalved inlet 26 is opened. This circumstance is represented by step 39of FIG. 3. Reducing gas post-flow is continued for a period of time,represented by step 41.

At step 43 reducing gas post-flow is stopped and at step 45 chamberpumpout is performed. At step 47 the slit transfer valve (element number28, FIG. 1) is opened, and the finished wafer is removed and a new waferis inserted at step 49. The slit valve is closed again after wafertransfer at step 51. Pumping speed is then throttled (step 53) andreducing gas pre-flow is immediately started at step 55. At step 56isolation valve 27 is opened again, and at step 57 process gas flow isstarted, and may continue at different levels and different gasesthrough all of the conventional process steps established until thedesired film is formed. Process flow then returns to step 37 whereprocess gas flow is again suspended at the end of processing.

Preflow of reducing gas, post flow of reducing gas, and use of theisolation valve to minimize outgasing from manifolds into the CVDreactor have all been demonstrated by the inventors to effectas-deposited film stress.

It is not only film stress that is affected in the practice ofembodiments of the invention. The inventors have also found that theheretofore unsuspected presence of WF₆ without adequate reducer gas atthe beginning and end of conventional processing is responsible for asignificant proportion of wormhole formation, as the WF₆ is reduced bythe silicon or polysilicon materials on the wafer. Moreover, practicingthe present invention, as it reduces or eliminates the hightungsten-content films found to be present at the interfaces of tungstensilicide films, also lessens deposition of high-stress tungsten-richmaterial on the walls of CVD reactor chambers, providing a cleanerprocess which can be repeated more often before chamber service isrequired.

Another effect of the presence of pre-and-post tungsten-rich films isformation of voids during oxidation processes that follow the depositionsteps discussed above. During such oxidation processes excess siliconatoms in the tungsten silicide film diffuse through the silicide film tothe surface, to take part in the oxidation reaction. If there isinadequate silicon available for oxidation, which is the case iftungsten-rich tungsten silicide is present either at the surface, or atthe polysilicon/silicide interface, then there is a tendency for 1) voidformation or 2) tungsten oxide formation.

Voids are nucleated at the interface of the polysilicon/silicide whenthe only available source of silicon for the oxidation process is theunderlying polysilicon. As silicon from the poysilicon is transported tothe surface through defects or pinholes in native oxide present on thepolysilicon, voids are created.

If the oxide at the polysilicon/silicide interface is intact and of acertain thickness which prevents silicon diffusion from the polysilicon,then there is a tendency for the tungsten in the tungsten silicide filmto form tungsten oxide during the oxidation process. Tungsten oxides areunstable and volatile, and often result in delamination of the tungstensilicide film.

Either case described above can be prevented by avoiding deposition of atungsten-rich film after deposition of tungsten silicide. Even moreeffective is a process wherein a silicon-rich outer surface layer oftungsten silicide is deposited, wherein the excess silicon contentmatches or exceeds that required by the oxidation process. For example,oxidizing silicon at 850 degrees C. for 30 minutes requiresapproximately 5E15 atoms/cc of excess silicon in the outer film layer.All of the silicon required for the oxidation process is locallyavailable in this scheme, and there is no driving force to cause siliconatoms to migrate from the underlying polysilicon or tungsten atoms fromthe tungsten silicide film.

In the art to date the problem of void formation has been addressed bydeposition in a separate process of polysilicon directly onto apreviously deposited tungsten silicon layer. It is also known in the artto deposit in-situ a film of polysilicon upon a layer of tungstensilicide to avoid void formation. In an embodiment of the presentinvention, however, in-situ steps are taken to ensure that the lastportion of a deposited tungsten silicide film is, in one aspect, nottungsten-rich, and, in another aspect is intentionally silicon-rich.

In the first case, to ensure that the outer layer of a depositedtungsten-silicide film is not tungsten-rich, the steps described aboverelative to FIG. 3 are followed, wherein one or more of the steps ofpre-flow of reducer gas, post flow of reducer gas, and use of anisolation valve are followed. In the second case, to add furtherassurance of avoiding void formation in subsequent oxidation processing,a silicon-rich surface to the tungsten silicide film is provided byincreasing the flow of reducer gas in the last portion of theconventional deposition of tungsten silicide. For example, assuming aratio of the silicon-bearing gas to WF₆ for conventional processing fora time t, in an embodiment of the present invention this ratio isincreased during a final portion of time t, perhaps 10%, to ensure thatthe final portion of the deposited film will be silicon-rich.

Further, the inventors have found particular advantage in practice ofthe invention in conjunction with formation of tungsten silicide filmsusing dichlorosilane. Manufacturers have found it desirable to decreaseanneal time and temperature as much as practical in large part becausevertical and lateral device dimensions have significantly decreased withincreases in device density in chip design. It is known thatafter-anneal resistivity decreases to a lesser extent as anneal time andtemperature are reduced, and that after anneal resistivity is a functionof as-deposited resistivity and silicon to tungsten ratio (Si:W).Unfortunately, however, the same steps that reduce as-depositedresistivity produce higher as-deposited stress, requiring higher annealtemperature and longer anneal time.

By practice of the present invention, Si:W may be reduced and othervariables managed to lower as-deposited resistivity while as-depositedstress is also kept low. Low as-deposited resistivity is provided byreducing chamber total pressure to about 250 milliTorr, which provides alow Si:W. In the anneal, as a consequence, less Si is required todiffuse out of the tungsten silicide film, therefore a lower than usualtemperature is required to complete the anneal.

It will be apparent to those with skill in the art that there are manyalterations and alternatives to the embodiments described above, withoutdeparting from the spirit and scope of the invention. For example, thereare many ways isolation valves may be implemented and positioned otherthan that depicted above. Moreover, there are several ways reducing gaspre-flow and post-flow may be provided. The lengths of time for suchflows of reducing gas will also vary depending on a number ofcircumstances, mostly related to particular equipment configuration.There are similarly many other alterations that might be made. Theinvention is limited only by the claims which follow.

What is claimed is:
 1. A method for depositing a tungsten silicide film on a substrate in a CVD reactor chamber, comprising steps of:(a) introducing WF₆ and reducer gas into the reactor chamber for a first period of time at a first ratio of reducer gas to WF₆ to accomplish deposition of the tungsten silicide film on the substrate: and (b) establishing a second ratio of reducer gas to WF₆ higher than the first ratio for a second period of time, ensuring thereby a silicon-rich layer on the tungsten silicide film.
 2. The method of claim 1 further comprising a step prior to step (a) for establishing a preflow of silicon-bearing reducer gas to the reactor chamber before the substrate attains coating temperature, thereby preventing preformation of tungsten-rich film on the substrate before planned process steps begin.
 3. The method of claim 1 comprising a further step following step (b) for continuing flow of the silicon-bearing reducing gas for a period of time after the WF₆ flow is stopped, thereby preventing postformation of tungsten-rich film on the silicon-rich layer deposited in step (b).
 4. The method of claim 1 wherein the CVD reactor chamber is used for repeated process cycles wherein one or more wafers are coated in one cycle, and a different wafer or wafers are coated in subsequent cycles, and wherein the CVD reactor chamber is connected by a manifold to a gas injection and flow control subsystem including a source of WF₆, and further comprising a step for closing an isolation valve in the manifold between the CVD reactor chamber and the gas injection and flow control subsystem for substantially all of the time between the end of one deposition cycle and the beginning of a subsequent deposition cycle.
 5. The method of claim 1 further comprising a step prior to step (a) for establishing a preflow of silicon-bearing reducer gas to the reactor chamber before the substrate attains coating temperature, thereby preventing preformation of tungsten-rich film on the substrate before planned process steps begin, and a step following step (b) for continuing flow of the silicon-bearing reducing gas for a period of rime after the WF₆ flow is stopped, thereby preventing postformation of tungsten-rich film on the silicon-rich layer deposited in step (b). 